1. Field of the Invention
The present invention relates to a semiconductor device having a memory transistor on a substrate and a manufacturing method thereof.
2. Description of the Related Art
In recent years, a semiconductor memory device such as a computer having a nonvolatile semiconductor memory element that stores data and program is becoming to be more used. A semiconductor memory device is typically formed of a memory cell array including a plurality of memory transistors in which data is stored (also referred to as memory cell transistors), a circuit for performing writing, erasing, or reading, and a decoder that determines a memory transistor according to an address signal. The elements that form these circuits are required to have different characteristics, therefore, the structures thereof vary accordingly.
An EEPROM (Electric Erasable Programmable Read Only Memory) and a flash memory are known as memories representing a semiconductor nonvolatile memory. When using a memory transistor having a floating gate electrode among these memories, a tunnel oxide film between a semiconductor layer and a floating gate electrode of a memory transistor is required to be as thin as possible. This is because the tunnel oxide film has to be thin enough to allow a tunnel current to flow therethrough. Moreover, a memory transistor is formed of two gate electrodes and two insulating films since a control gate electrode and an insulating film sandwiched by the control gate electrode and the floating gate electrode are included in addition to the floating gate electrode and the tunnel oxide film.
A transistor that forms a decoder is required to operate at a high speed, thus it is preferable that a gate insulating film thereof be thin.
On the other hand, in the circuit for performing writing, erasing, or reading, a voltage as high as a voltage applied to a memory transistor is applied to a transistor that forms the circuit, therefore, a gate insulating film is required to be thick so that the transistor is not broken in such a situation.
As described above, a semiconductor memory device is formed by integrating elements of different structures.
As a related art, there is a technique for forming an active matrix substrate in which a pixel portion, a driver circuit portion, and a memory portion are integrally formed on the same substrate. In this case also, transistors of different structures are integrally formed on an active matrix substrate. Specifically, a gate insulating film of a transistor in the memory portion is formed thinner than a gate insulating film of the pixel portion and the driver circuit portion in order to manufacture a transistor having a structure according to each function (see Patent Document 1).
[Patent Document 1]
Japanese Patent Laid-Open No. 2000-356788